Power consumption associated with the transmission of wireless signals (e.g., Wi-Fi, cellular, Bluetooth, etc.) may quickly drain the battery of a mobile device. Thus, it is desirable to reduce the power consumption of transmitters in mobile devices.
FIG. 1 is a block diagram of a conventional direct-conversion transmitter 100 using quadrature amplitude modulation (QAM). Transmitter 100 includes an antenna ANT, a baseband processor 110, and an analog front end (AFE) 120. The AFE 120 includes a digital-to-analog converter (DAC) 121A for the I signal path, a filter 122A for the I signal path, a local oscillator (LO) mixer 123A for the I signal path, a DAC 121B for the Q signal path, a filter 122B for the Q signal path, an LO mixer 123B for the Q signal path, a combiner 124, and a linear power amplifier (PA) 125. The mixers 123A and 123B up-convert the I and Q signals from baseband directly to the carrier frequency by mixing the I and Q signals with local oscillator signals LO(I) and LO(Q), respectively, where the frequency of the local oscillator signals is the carrier frequency. The combiner 124 combines the up-converted I and Q signals, and the PA 125 amplifies the combined I/Q signal for transmission as TX via the antenna ANT.
If the PA 125 is an op-amp (or another type of linear amplifier), then the PA 125 may be well suited for transmitting signals using various amplitude modulation techniques (e.g., OFDM) for which changes in the output signal should be proportional to changes in the input signal. However, linear amplifiers (e.g., the PA 125) consume a significant amount of power.
Although switched-mode output drivers consume less power than linear amplifiers such as PA 125, switched-mode output drivers are non-linear devices that may not be suitable for transmitting OFDM symbols. For example, because OFDM techniques are associated with higher peak-to-average ratios (PAR) than zero-PAR (GMSK) or low-PAR modulation techniques, switched-mode output drivers may not be able to achieve sufficient driver linearity for OFDM techniques. Further, because OFDM techniques typically use high frequency signals, driving switched-mode output drivers at such high rates may not be feasible because of limitations of switching speeds of switched-mode output drivers.